Using Acid/Base Neutralization to Study

Endothermic vs. Exothermic Reactions and Stoichiometry

Overview

The chemical reaction of interest in this lab is:

H2SO4(aq)+ 2NaOH(aq) Na2SO4(aq)+ 2H2O(l)

You will be answering the following questions:

  1. Is the reaction Endothermic or Exothermic?
  2. What is the Limiting Reactant at various times in the procedure?
  3. What is the concentration of the H2SO4 solution?

This will be accomplished by first gradually adding H2SO4(aq) to 25.0cm3 of NaOH(aq) and monitoring how the temperature changes while you do so. You will then perform some simple calculations to get your experimental data into a more useful form and graph it. The graph will be used to answer the first two questions above. Finally, the third question will be answered with the aid of some algebra!

The following materials and equipment will be available at your Lab Station:

  • Safety goggles
  • Latex gloves
  • H2SO4(aq) solution (Unknown Concentration)
  • NaOH(aq) solution (2.0M)
  • 250cm3beaker
  • 25cm3 pipette
  • 100cm3 burette
  • Styrofoam cup
  • Thermometer
  • Graph paper
  • Ruler
  • Calculator
  • Lab report document

For simplicity, we will refer to theNaOH(aq) and H2SO4(aq) solutions as A and B, respectively.

Staging the Experiment

  1. Before doing anything, make sure you have the proper safety equipment and your environment is free of trip hazards and other potentially dangerous situations. Please ask your instructor any questions you have to ensure that you and everyone around you has a safe learning experience.
  2. Construct your reaction vessel.
  • Place the Styrofoamcup into the 250cm3 beaker. The cup should not fall the entire way into the beaker; it should be suspended by the rim.

  1. Construct yourdata table.
  • A blank data table has been provided on the following page. You will be using this table to record your data as the experiment proceeds. Many of the blocks have already been filled in to help guide you through the experiment. At this time, however, you should fill in the units for each quantity specified in the column headers. Write them in the appropriate parentheses. (The variables are defined below the table.)
  1. Familiarization:
  • Review the remainder of the experimental procedures to ensure that you are prepared. Remember to ask yourinstructor to clarify any questions you might have.
  1. Check to be sure all the materials listed above are available.
  2. Fill the burette with at least 60cm3 of H2SO4(aq). Remember, we will be calling this solution B.

Experimental Procedure

  1. Establish the initial temperature of the NaOH(aq) solution. Remember, we will be calling this solution A.
  1. Transfer 25.0cm3 of A to the reaction vessel using the pipette.
  2. Record a value of “0” in the VB column of the table.
  3. Use the thermometer to determine the temperature of the liquid in the reaction vessel and record the temperature in the appropriate place in the data table. (In this case, the appropriate line in the table is the one with a 0cm3 target for B because none was added.) Please note that this temperature will be referred to as T1 in the future.
  1. Collect the remainder of the data.
  1. Move the reaction vessel under the burette containing solution B and add approximately 5.0cm3 of B.
  2. Stir gently and use the thermometer to measure the new temperature.
  3. Record the volume of solution B added and the new temperature in the data table.
  4. Repeat until all the trials specified in the data table have been completed.

Data Table

Run # / VA
(cm3 ) / VB(Target)
(cm3) / VB(Added)
( ) / T
( ) / VBT
( ) / VT
( ) / T
( ) / VTT
( )
1 / 25.0 / 0
2 / 25.0 / 5.0
3 / 25.0 / 5.0
4 / 25.0 / 5.0
5 / 25.0 / 5.0
6 / 25.0 / 5.0
7 / 25.0 / 5.0
8 / 25.0 / 5.0
9 / 25.0 / 5.0
10 / 25.0 / 5.0
11 / 25.0 / 5.0

VA = volume of Solution A

VB(Target) = Target volume of Solution B to be added

VB(Added) = Actual Volume of Solution B added

T = Temperature of Solution

VBT = Total volume of Solution B added

VT = VA + VBT

T = T – T1

T1 = Temperature for Run 1 (Run with VB(Added) = 0)

VTT = VT * T

Analysis I: Data Transformation

The data table now contains your experimental results. The next step is to perform a series of basic calculations to convert the recorded values into a form which will simplify the analysis. Fill the rest of the table using the equations found at the bottom of the table.

Analysis II: Graphical Analysis

Using the graph paper provided, plot your result.

x-axis: VBT

y-axis: VTT

If everything went according to plan, your data shouldbe grouped in two linear regions with distinctly different slopes, one positive and the other negative. Using your ruler, draw a line of best fit for each of the two regions.

The lines you have just drawn will intersect. Record the VBT value of the intersection point below (remember to include units). We will refer to this quantity as VBN in the future.

VBN =…………………..

Analysis III: Implications

Using your graph, explain why you believe there are regions that exhibit different behaviors. (Limit your answer to no more than 4 sentences.)

What do you believe is the significance of VBN?

Is the reaction endothermic or exothermic? Why?

What is the limiting reactant in the portion of the graph where the slope is positive?

What is the limiting reactant in the portion of the graph where the slope is negative?

Analysis IV: Stoichiometric Analysis

You will now use the results of your graphical analysis to determine the concentration of Solution B, the H2SO4(aq).

In order to perform this calculation, you should refer to the equation for this neutralization reaction:

H2SO4(aq)+ 2NaOH(aq) Na2SO4(aq)+ 2H2O(l)

You must also realize that VBN is the volume of H2SO4(aq) required to neutralize 25.0cm3 of NaOH(aq) (2.0M). Show your work!

Concentration of H2SO4(aq) = ______

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